cook the vegetables for meat sauce
Place a large skillet over medium-low heat. Add the olive oil, lard (optional), finely chopped bacon, finely chopped onion, finely chopped celery stalk, finely chopped bell pepper, and a sprig of thyme. Cook for about 3 minutes, stirring occasionally. Add the finely chopped garlic and cook for another 3 minutes.
Increase the heat, add the ground beef and cook for 3 - 5 minutes, stirring occasionally. Add the tomato paste, diced tomato, tomato purée, beef stock or water and stir to combine. Reduce the heat and season with bay leaf, salt, and pepper. Cover with a lid and cook for 45 - 50 minutes. Stir the heavy cream into the meat sauce the last 10 minutes.TipHeavy cream will make the sauce creamy and richer.
Using a mandoline, slice the zucchini into 2 mm or 1/16 inch thick slices. Place a grill pan over high heat. Add the zucchini noodles and grill them for about 1 minute on each side. Make sure they aren't too crowded. Transfer to a clean plate. Drizzle the zucchini noodles with olive oil. Add chopped parsley, chopped garlic, and season with salt and pepper. Set aside for about 10 minutes for zucchini to marinate. Then squeeze the zucchini to release any excess water.
Add butter and ricotta to a saucepan. Place over low heat. Stir to combine and cook for a minute, then add in the heavy cream, 1/2 tsp nutmeg powder, bay leaf, and season with salt and pepper. Bring to a boil (still on low heat), then immediately remove from the heat and stir in the shredded Pecorino Romano cheese and shredded Gouda cheese. Stir using a wire whisk until the cheese melts and the cheese sauce is thick and creamy. Add the baking powder and egg yolk. Stir to combine.
Assemble Zucchini Lasagna
To assemble the zucchini lasagna, prepare a 25 cm x 20 cm or 8 x 10 inch baking dish. Start with a layer of ground beef meat sauce (about 3 tbsp), followed by a layer of zucchini, meat sauce, cheese sauce, then again a layer of zucchini, meat sauce, cheese sauce. Continue with the layering until you have 4 layers of each component. The last layer should be cheese sauce topped with shredded Gouda cheese. Cover with parchment paper and aluminum foil (make sure the foil doesn't touch the food). Place in a preheated oven. Bake for 10 minutes at 190 °C or 375 °F (covered), then remove the foil and parchment paper and bake for additional 10 - 15 minutes at 200 °C or 390 °F.
Remove the baked Zucchini Lasagna with Ground Beef from the oven and let stand for 5 - 10 minutes before serving. Store any leftovers in an airtight container in a fridge for up to 2 days.